1. Field of the Invention
This invention relates generally to methods and apparatus for measuring and controlling fluid flow rates and, more particularly, to a method and apparatus for dispensing natural gas.
2. Brief Description of the Prior Art
Over the past few years, there has been a steadily increasing interest in developing alternative fuels for automobiles in an effort to reduce the harmful emissions produced by conventional gasoline and diesel powered vehicles. One such alternative fuel that has already been used with favorable results is compressed natural gas (CNG). Besides being much cleaner burning than gasoline or diesel fuel, most modern automobiles can be converted to operate on compressed natural gas (CNG). Typically, such a conversion may include various minor modifications to the engine and fuel delivery system and, of course, the installation of a natural gas fuel tank capable of storing a sufficient amount of CNG to provide the vehicle with range and endurance comparable to that of a conventionally fueled vehicle. In order to provide a reasonably sized storage tank, the CNG is usually stored under relatively high pressures, such as 3,000 to 4,000 pounds per square inch gauge (psig).
While the conversion process described above is relatively simple, the relatively high pressure under which the CNG is stored creates certain refueling problems that do not exist for conventional vehicles powered by liquid fuels, such as gasoline. Obviously, since the gas is transferred and stored under high pressure, special fittings, seals, and valves have to be used when the CNG is transferred into the CNG storage tank on the vehicle to prevent loss of CNG into the atmosphere. Also, special precautions must be taken to minimize the danger of fire or explosion that could result from the unwanted escape of the high pressure CNG. Accurate, yet convenient and easy to use measurement of the amount of CNG delivered into the vehicle's storage tank is also a problem. Consequently, most currently available natural gas refueling systems require that several relatively complex steps be performed during the refueling process to prevent leakage, minimize the risk of fire or explosion, and to measure the amount of fuel delivered. Unfortunately, because such processes tend to be relatively complex, they cannot be carried out very easily by most members of the general public or even by unskilled workers. Therefore, most CNG dispensing systems usually require trained personnel to perform the refueling process. As of date, providing trained operators to perform the refueling operation has not yet posed a significant problem, because natural gas refueling stations are generally limited to fleet operators of vehicles who can afford to have trained personnel to perform the refueling operations and who either do not care to keep accurate measurements of each vehicle fillup or who can afford complex flow measuring equipment to do it. However, because the interest in natural gas powered automobiles is increasing rapidly there is a growing need to develop a natural gas refueling system that is highly automated and has sufficient fail-safe systems to minimize the danger of fire or explosion, while at the same time being capable of accurate measurements and being used safely by the general public. Ideally, such a natural gas dispensing system should be as familiar to the customer and as easy to use as a conventional gasoline pump and refueling station.
As mentioned above, there are several natural gas dispensing "pumps" currently available. One such system is disclosed in the patent to Fisher et al., U.S. Pat. No. 4,527,600. While the dispensing system disclosed by Fisher et al., is relatively easy to use, it requires certain relatively expensive components. For example, Fisher's dispensing system utilizes differential pressure transducers to determine the amount of CNG that is dispensed into the vehicle tank. Disadvantageously, however, such differential pressure transducers are expensive, and have a rather limited range of operation of about 3 to 1.
Another significant problem associated with the dispensing systems currently available, such as the system disclosed by Fisher, is that such systems cannot determine accurately when the natural gas storage tank in the vehicle is filled to rated capacity, yet not overfilled. That is, since natural gas storage tanks in vehicles have to be rated to safely contain CNG under a given pressure at a given temperature (e.g., 3000 psig at a temperature of 70.degree. F., the "standard temperature"), it is important to determine the correct pressure to which the tank should be filled when the ambient temperature is not exactly 70.degree. F. For example, if the ambient temperature is warmer than the standard temperature of 70.degree. F., the tank can be filled safely to a pressure higher than the rated pressure. In fact, the tank will not be completely filled under such circumstances until it is at such a higher pressure. Conversely, if the ambient temperature is below standard temperature, the tank cannot be filled safely to the rated pressure, because as the CNG warms to the standard temperature, the pressure will exceed the rated pressure. In this situation, the tank is overfilled, and there is a significant danger of the safety relief valve on the tank venting the excess CNG to the atmosphere, thereby losing the CNG and possibly even creating an explosion hazard. Worse yet, the tank may actually rupture if the safety valve malfunctions.
Unfortunately, however, none of the currently available natural gas dispensing pumps compensate for changes in ambient temperature. Accordingly, these currently available dispensing systems are usually configured to turn off the flow of natural gas at pressures well below the rated pressure of the tank to avoid the dangerous overfilling and consequent over-pressurization the vehicle storage tank described above. Consequently, if the ambient temperature is higher than the standard temperature of 70.degree. F., the tank will be substantially underfilled.
Another problem relates to accurately sensing the vehicle tank pressure while the vehicle tank is being filled. For example, it is impossible to sense the vehicle tank pressure with a remotely located pressure sensor if the CNG flow through the dispensing hose reaches sonic velocity (a choke point) at some point between the pressure sensor and the vehicle tank itself. Typically, such a choke point occurs in the safety check valve located in the vehicle tank coupler assembly. Accordingly, such dispensing pumps are usually designed to ensure that the flow of CNG between the remote pressure sensor for sensing the vehicle tank pressure and the vehicle tank itself remains subsonic at all times and under all flow conditions, which, of course, limits the maximum delivery rate of the pump. Unfortunately, even if the dispensing pump is designed to ensure that a sonic choke point does not occur between the pressure sensor and the tank, it is still necessary to compensate for pressure errors due to the pressure drop in the hose and coupler/check valve assembly, which is difficult, since the pressure drop in the vehicle check valve may vary depending on the characteristics of particular valve.
Therefore, there is a need for a natural gas dispensing system that provides the desired degree of safety for dispensing natural gas under high pressures that is preferably as easy to use a conventional gasoline pump. Such a dispensing system should be relatively simple and reliable and ideally would not require expensive and complex differential pressure transducers. Most importantly, such a dispensing system should be capable of automatically determining a temperature corrected cut-off pressure to ensure that the vehicle storage tank is completely filled regardless of the ambient temperature and regardless of whether the CNG flows through a sonic choke point in the dispensing hose or coupler/check valve assembly. Finally, it would be desirable for such a dispensing system to accommodate two or more dispensing hoses from a single supply plenum to reduce the number of pressure and temperature sensors to a minimum, thus providing better overall system reliability and lower cost.